1. Field of the Invention
Applicant's invention relates to a method and apparatus for teaching students the principles of structural analysis using nuclear magnetic resonance (NMR) spectroscopy.
2. Background Information
Nuclear magnetic resonance (NMR) spectroscopy is used for the study of molecular structure through measurement of the interaction of an oscillating radio-frequency electromagnetic field with a collection of nuclei immersed in a strong external magnetic field. These nuclei are parts of the atoms that are assembled into these molecules.
Once the NMR spectrum is obtained, the determination of the unknown structure is based on three requirements indicated on the spectrum. These three requirements are integration, splitting due to spin-spin coupling, and chemical shift. Each anticipated chemical fragment is determined from the chemical shift on the spectrum. Chemically different hydrogens in a molecule do not experience the same magnetic field. Electrons shield the nucleus thereby reducing the effective magnetic field and requiring energy of a lower frequency to cause resonance. On the other hand, when electrons are withdrawn from a nucleus, the nucleus is deshielded and feels a stronger magnetic field requiring more energy (higher frequency) to cause resonance. Thus, the NMR spectrum can provide information about a hydrogen's electronic environment. Generally, hydrogens bound to carbons attached to electron withdrawing groups tend to resonate at higher frequencies (more downfield, to the left of the spectrum) from TMS, tetramethylsilane, a common NMR standard. The position of where a particular hydrogen atom resonates relative to TMS is called the chemical shift.
Integration is the second item that can be determined from an NMR spectrum. For the integration, the area under the NMR resonance is proportional to the number of hydrogens that contribute to that resonance. In this way, by measuring or integrating the number of different NMR resonances, information concerning the relative number of chemically distinct hydrogens can be obtained. Experimentally, the integrals often appear as a line over the NMR spectrum. Integration only gives information on the relative number of different hydrogens on the represented chemical fragment, not the absolute number.
The last item of information that can be determined from the NMR spectrum is splitting. The spectrum provides information on how many hydrogen neighbors exist for a particular hydrogen or group of equivalent hydrogens. In general, an NMR resonance will be split into N+1 peaks where N is the number of hydrogens on the adjacent atom or atoms. If there are no hydrogens on the adjacent atoms, then the resonance will remain a single peak, a singlet. If there is one hydrogen on the adjacent atoms, the resonance will be split into two peaks of equal size to form a doublet. Two hydrogens on the adjacent atoms will split the resonance into three peaks with a ratio of 1:2:1 being a triplet. If there are three hydrogens on the adjacent atoms, the resonance will split into four peaks with an area in the ratio of 1:3:3:1 forming a quartet.
When a student is first introduced to these concepts in an organic chemistry course, he or she does not typically have difficulty determining the identity of an unknown molecule as long as the molecule remains fairly simple, such as a molecule having only a few carbons. However, as the molecules become larger and multiply branched, structural determination by the student becomes quite difficult if not impossible.
Every full year organic chemistry text includes a chapter or half a chapter on NMR spectroscopy. Subsequent chapters then include practice problems involving NMR interpretation. NMR in these texts is taught the same way. First the authors start with a molecule and explain its spectrum. This is done for several molecules pointing out the chemical shifts, integration and splitting patterns. Several texts point out common patterns, but most leave it to the students to figure out how to go from the spectrum to the molecule. This is a much more difficult process. In some texts, some simple rules are given such as (1) count the number of signals which is equal to the number of types of hydrogens, (2) figure out the chemical fragments from the chemical shifts, (3) and solve the problem.
While this can work for simple molecules it is virtually guaranteed to fail for more complex spectra. Unfortunately, there are currently no “hands-on” educational tools available to assist students with molecular structure identification from NMR spectra, particularly complex spectra. The present invention satisfies this need for a “hands-on” NMR educational tool that can assist students in NMR structural analysis.